[tool search] support namespaced deferred dynamic tools (#18413)

Deferred dynamic tools need to round-trip a namespace so a tool returned
by `tool_search` can be called through the same registry key that core
uses for dispatch.

This change adds namespace support for dynamic tool specs/calls,
persists it through app-server thread state, and routes dynamic tool
calls by full `ToolName` while still sending the app the leaf tool name.
Deferred dynamic tools must provide a namespace; non-deferred dynamic
tools may remain top-level.

It also introduces `LoadableToolSpec` as the shared
function-or-namespace Responses shape used by both `tool_search` output
and dynamic tool registration, so dynamic tools use the same wrapping
logic in both paths.

Validation:
- `cargo test -p codex-tools`
- `cargo test -p codex-core tool_search`

---------

Co-authored-by: Sayan Sisodiya <sayan@openai.com>

codex-rs/tools/src/dynamic_tool_tests.rs

@@ -8,6 +8,7 @@ use std::collections::BTreeMap;
 #[test]
 fn parse_dynamic_tool_sanitizes_input_schema() {
     let tool = DynamicToolSpec {
+        namespace: None,
         name: "lookup_ticket".to_string(),
         description: "Fetch a ticket".to_string(),
         input_schema: serde_json::json!({
@@ -42,6 +43,7 @@ fn parse_dynamic_tool_sanitizes_input_schema() {
 #[test]
 fn parse_dynamic_tool_preserves_defer_loading() {
     let tool = DynamicToolSpec {
+        namespace: None,
         name: "lookup_ticket".to_string(),
         description: "Fetch a ticket".to_string(),
         input_schema: serde_json::json!({